There are conventionally known airbag devices for a front passenger's seat that have an airbag, an inflator that supplies gas to this airbag, and a case that accommodates the airbag and the inflator, and that are provided at the upper portion of the region in front of the front passenger's seat of an instrument panel. Further, among these airbag devices for a front passenger's seat, there are those in which the case has a rear side accommodating portion that accommodates the lower inflating portion of the airbag, and a front side accommodating portion that accommodates the inflator and the general portion, other than the lower inflating portion, of the airbag (see, for example, Japanese Patent Application Laid-Open No. 2009-179267).